Measurements have been made by Hodge of ultrasonic velocities at 27°C at pressures from 1 to 100 atmos. in air and N2 and at 1 to 60 atmos. in CO2. The results for air and N2 combined with the respective compressibility data of Holborn and Otto give at 27°C for air, values of γ between 1.406 at 1 atmos. and 1.580 at 100 atmos., and for N2, 1.403 at 1 atmos. to 1.564 at 100 atmos. The acoustic velocities in CO2 combined with the compressibility data of Amagat, give for γ at 27°C values from 1.304 at 1 atmos. to 3.524 at 60 atmos. These and the other results between these limits are in excellent general agreement with the few results in this temperature and pressure region available for comparison.